Hybrid fiber coaxial (HFC) cable television systems include a so-called headend which provides communications between end users in the HFC network and IP/PSTN networks. The headend typically contains a cable modem termination system (CMTS) containing several receivers, each receiver handling communications between hundreds of end user network elements. An example of a CMTS is the Motorola Broadband Service Router 64000 (BSR 64000).
The headend is generally connected to several nodes and each node is connected to many network elements. Examples of network elements include cable modems, set top boxes, televisions equipped with set top boxes, data over cable service interface specification (DOCSIS) terminal devices, media terminal adapters (MTA), and the like. A single node may be connected to hundreds of modems.
A typical HFC network uses optical fiber for communications between the headend and the nodes and coaxial cable for communications between the nodes and the end user network elements. Downstream (also referred to as forward path) optical communications over the optical fiber are typically converted at the nodes to RF communications for transmission over the coaxial cable. Conversely, upstream (also referred to as return path) RF communications from the network elements are provided over the coaxial cables and are typically converted at the nodes to optical communications for transmission over the optical fiber. The return path optical link (the optical components in the HFC network, e.g. the transmission lasers, optical receivers, and optical fibers) contribute to the performance of the HFC network. More particularly, the optical components contribute to the quality of signals received by the CMTS from the network elements, and may cause distortion of the signals or otherwise degrade their quality.
The RF signals are generally in the form of modulated RF signals, and several modulation schemes exist with different levels of complexity. The use of the highest order modulation scheme is typically desired; however, complex modulation schemes are generally more sensitive to impairments. For instance, a given amount of phase noise or narrow band interference may prevent an HFC network from effectively using preferred modulation schemes because phase noise and/or narrowband interference often limits the level of modulation complexity that may be transmitted over an upstream HFC plant.
A phase noise impairment is generated through modulation and demodulation processes between the DOCSIS terminal devices (MTA or cable modems) and the CMTS. This noise generally combines with and potentially degrades the noise already present within the downstream or upstream HFC plant. In contrast, a distortion-based impairment is typically generated by non-linear active components in the forward or return path. Distortion may produce appreciable narrowband and/or wideband spurious signals which can interfere with frequencies used by the CMTS and DOCSIS terminal devices.
An accurate diagnosis of phase noise and/or distortion-based impairment issues has conventionally required technicians and/or engineers to be positioned at multiple locations within the HFC plant to simultaneously inject test signals at suspected DOCSIS terminal device locations so that the performance of the network can be accessed at the headend location with specialized test equipment, such as a vector signal analyzer. The problem with this diagnostic approach is that it is manual, time consuming and expensive.